Bilateral video transmission system



358-86. XR. 299749188 '"m WM Mfh l, 1951 H. M. DIAMBRA 2,974,188 MJM i @w I BILATERAL VIDEO TRANSMISSION SYSTEM u,

Filed Deo. 19, 1956 '3 Sheets-Sheet 1,- f

Fia-i INVENTOR ATTORNEY H. M. DIAMBRA BILATERAL VIDEO TRANSMISSION SYSTEM March 7, 1361 3 Sheets-Sheet 2 Filed DBC. 19, 1956 VO PM2 mmPmOO m INVENTOR ATTORNEY 5 SheetsfSheet 3r Filed Dec. 19, 1956 FIG. 3

FIGA

digg fw @Wm l @l INVENTOR ATTOKNI' autres ljatented ily/lar. 7', 295i BXLA'IERAL VIDEO TRANSYSSION SYSTEM Henry M. Diainbra, Silver Spring, Md., assigner to Citizens Bank of Maryland, Riverdale, Md., and Small Business Administration, Richmond, Va.

Filed Dec. 19, 1956, Ser. No. 629,378

i1 Claims. (Cl. l78-S) The present invention relates to bilateral. communication circuits and more particularly to a closed-loop television system permitting of the simultaneous bilateral communication (both audible and visual) between television transmitters and receivers over a unitary transmission line.

In recent years there has been a considerable increase in utilization of closed-loop, radio-frequency television circuits, for example, by industry and the banking and medical professions to provide rapid visual and aural communication between remote locations. The cost of the prior art closed loop television systems has been quite high due in part to thc fact that bilateral communication required two complete and separate unilateral communication systems, each for one direction of communication. Specifically, where bilateral transmission was required, a television transmitter and receiver was disposed at each of the locations between which communication was desiredv and :i separate coaxial line with its associated booster equipment or equipments interconnected a receiver and a transmitter situated at opposite ends of the communication linkA Since expensive coaxial cables and amplifiers (in the booster circuits) must be employed at the frequencies involved in television communication, thc cost of the system was excessive due to the necessity for two distinct unilateral channels for bilateral communication.

In accordance with the present invention a single caxial transmission line may be employed for bilateral communication between remote locations in consequence of novel isolation circuits employed in the booster equipment and in the terminal equipments at the two ends of the transmission link and further in consequence of a novel arrangement of high and low pass filters in the booster equipment. The booster equipment employs a single broad band amplifier for amplifying information at all frequencies employed in the system and a novel arrangement of band pass filters for routing and prcventinfr interaction between the information signals l 'ing in` distinct frequency bands and proceeding in opposite directions along the cozutial transmission linc. Specitically, the booster equipment or equipments disposed at spaced locations intermediate the terminal equipments, employ a four-arm impedance bridge network having two pairs of opposed arms, two input terminals each connected to a distinct section of coaxial line and two conjugate terminals having a broad band radio frequency amplifier connected therebetween. mation proceeding in one direction are grouped in a first frequency bund and the several channels of information proceeding iu thc opposite direction are vgrouped in n second frequency band distinct from the lirsr. A first pair of opposed arms of the be" lge cach includes :i band pass tiltcr olleriug :i low impedance to the one frequency band and a high impedance to the other frequency band while thc second pair of opposed arms euch includes a band pass lilter ollering a high imperi-.nice to the one frequency band and a low impedance to the. other frelflic several channels of inforquency band. In one specific embodiment of the present invention, a three terminal, directional line splitting coupier, which may be of the type disclosed and claimed in copending patent application Ser. No. 400,256, filed on December 14, 1953, nov.' Patent No. 2,782,379, by Henry M. Diambra and George G. Edlen, for Directional Line Splitting Coupler, may be employed at each junction of two arms of thc bridge and the coaxial line and at each junction of two arms of the bridge and the input and output circuits of the broad band amplicr. The directional coupler provides a low impedance path between a first terminal and each of the two other terminals and a high impedance between the two other terminals. The couplers are connected to provide high impedance between the two arms of the bridge at each junction and to provide low impedance between each arm and the coaxial line or the aniplier circuit. The bridge arrangement provides a distinct low impedance path through the broad band ainplilier for information signals in each frequency band; only a very small portion of the signals in one frequency band appearing in the path provided for the other frequency band due to the high impedance pro vided by the filters at the former frequencies and further due to the high impedance between adjacent arms provided by the directional couplers. This feature of the present invention; that is, the high attenuation of signals of one frequency band in the path through the network for the other frequency band, permits the utilization of a single amplifier for signals in both frequency bands. The signals entering the booster network having just proceeded through a thousand or more feet of coaxial cable and are of quite low intensity. The amplifier therefore must have a high coin to amplify the incoming f3-s1 sut'clciently toproduce amplified signals capable of detection at the end of another thousand or more feet of coaxial linking the network with the terminal equipment or a further booster network. The amplified signals are appli-3d to the coaxial cable at e junction i the arms cf the network, as previously described, at which signals proceeding in the opposite direction over the transmission link are applied to the booster network. Therefore, a closedeloop is provided comprising'the amplifier, the arm of the bridge between the output circuit of the amplifier and the coaxial lead, and the arm of thc bridge between the coaxial lead and the input circuit of the amplifier. lf thgain of this closed-loop is unity or greater, the circuit would ring vand would obviously be inoperative. However, as :i result of the attenuation provided between the arms of the bridge by the directional couplers and the further attenuation of the information signals in one frequency band by the filter tuned to the other frequency band and connected between the coaxial cable and the input circuit of the amplifier, the gain of the aforesaid closed-loop is maintained at less than unity; thereby preventing ringing ot the circuit. Consequently, a single broad bund amplifier may be employed for amplifying the information in distinct frequency bands and proceeding in opposite directions through the transmission link and the booster network may be employed in a. single coaxial cable transmission link for bilateral multichannel comniunication.

As in all transmission line work, the coaxial lines and fillers must bc terminated in their characteristic impedance. ln the specific embodiment of the invention under consideration; that is, in :i system employing direcA tional couplers, the transmission lines connected to the booster networl-'s toed itl'. pt fer thro coupler to thc two baud pass e sf The band pass filter which accepts the KF. energy in the frequency bund of the incoming signals provides c. termination for the line at its characteristic impedance while the inipedance of the tilter tuned to a different band presents an impedance many times that of the characteristic impcdance. The directional coupler is a power divider circuit and the power applied over the coaxial line divides in an inverse ratio to the terminal impedances presented by thc two filters. ln consequence, only a small proportion of the input power is directed to the high impedance filter and this filter has the eflect of' a large impedance shuuting the lines characteristic impedance. The total impedance seen by the line is only slightly less than its characteristic impedance and the VSWR is maintained within acceptable limits.

The filter of the booster equipment feeding the coaxial line is even more nearly terminated in its characteristic impedance than the coaxial line due to the additional attenuation between. the two filters provided by the directional coupler. ln a specific example, the directional coupler may provide 3 db of, attenuation between the coaxial line and cach of the filters while providing db of attenuation between the two filters. The additional 17 db or" attenuation efectively increases the impedance shunting the characteristic impedance' presented by the coaxial line and therefore total impedance closely approximates the characteristic impedance of the filter.

In a second embodiment of the invention, the direc` tional couplers may be eliminated and the filters are modified to perform the functions of the couplers relating to preventing ringing of the booster circuit and providing termination of the coaxial lines and the filters in their characteristic impedance. In order to perform the function of the couplers relating to the preventing of ringing of the booster circuit, the attenuation of the filters need merely be increased to compensate for the reduced attenuation resulting from removal of. the coupiers. Thus if the couplers provide 2O db of attenuation between the filters in cach loop, the attenuation of the filters in their rejection must be increased by a me. ...ammi

The modification of the filters required to provide termination of the filters and coaxial lines in their characteristic impcdances depends upon the type of band pass filters employed. The second embodiment of the present invent n is described as employing series m band pass filters which are terminated at their input and output ends in series m half sections. Taking for example the junction of thc high and low pass filters and the coaxial line. the coaxial line -is connected to the low pass filter and the high pass filter respectively through aninduetor and a capacitor; that is, the first shunt impedance of each filter is eliminated, and a series connected inductor and capacitor are connected across the line. The series-circuit and thc inductor to the low pass filter and the capacitor to the high pass filter define the series m half section referred to above and provide proper termination or the coaxial line and of the filter feeding the line. lu the pass band of either filter the impedance presented atthe coax-ia! line is the mid-sliunt impedance of a series uz derived filter and in consequence closely approximates a fixed resistance at the value of the characteristic impedance. Stated differently the one filter in its rejection band furnishes the impedance required by the other filter in its pass band to present the characteristic impedance to the line or the filter feeding the line.

An identical arrangement may be provided at the` junction of the filters at the amplifier circuit. ln both enibodiments of thc invention` impedance transformation circuits, which muy take one ol' many known forms such as attenuation networks or impedance matching ti'uns l'ormcrs, must be employed to match the input or output impedance of t amplifier to the cl.; "istie irripetiunee of the lihcrs, if the filters feed or nre fed by the amplifier directly or to the characteristic impedance of a coaxial cable connected between the junction of the filters ond the amplifier circuit.

if between two sets of terminal equipments located at opposite ends of a transmission link.

Each terminal equipment includes at least one television transmitter and one television receiver, the transmitter at one end of the line generating signals in a frequency band to which the receiver at the other end of; the line is tuned and the transmitter and receiver at the same end of thc line being tuned to distinct frequency bands. The transmitter at one end of the transmission line must produce signals ot suflicient intensity to be detectable at the booster equipment after traveling over and being attenuated by long lengths of coaxial cable. The signals received at this end of the line on the other hand, are of low intensity having been attenuated by the long length of coaxial cable connected between the terminal equipment and the booster circuit. The high intensity signals produced by a transmitter at one end of the line cannot produce ringing in the receiver located at the same end OE the line since the receiver is tuned to a different frequency baud. The transmitted signals; how ever, can. overload or actually swamp the receiver and produce serious distortion of the low intensity incoming signals. In order to overcome this problem at the terminal equipment the transmitter is connected through a band pass filter, tuned to its frequency band, to one ter.

minal of a directional coupler, the receiver is connected through a filter tuned to its frequency band to another crininal of the directional coupler and the third terminal of thecoupler is connected to the coaxial lead. The attenuation provided by the directional coupler and a properly designed receiver filter may be made suflicient to prevent overloading of the receiver by the transmitted signals. As in the booster network, the directional couplers may be replaced by carefully designed band pass filters. With respect to both the terminal and booster equipments, the circuits employing directional couplers are preferred the tu. of the :rc-sity reduces the complexity and cost of the hlter s and the design problems incident thereto. The highly-attenuated, locally generated signals appearing at the receiver are, however, of sufiicient intensity to be detected by a receiver tuned to their frequency and a local monitor may be employed to detect these signals and provide at the transmitter a visual and audible reproduction of thc transmitted information for guidance of personnel utiliz ing the transmitter.

The terminal equipment has thus far been described as employing only a single transmitter and a single receiver but it is within the scope of the present invention to employ multiple transmitters and receivers at each terminal equipment. The transmitters at cach end of the television link transmit signals over distinct but adjacent video channels which fall within the common frequency band or"- their common band pass filter both at the terminal equipment and in the booster network. Similarly, multiple receivers at each end of the. line may be tuned to distinct but. adjacent video channels having fre quencics falling within the pass band of their associated filters. Consequently, the apparatus operates successfully for one or more television channels in euch direction of rnnsmision as longr as the frequencies of the signals proceeding in a common direction fall within the passv band' of the filters. l

Directional couplers are employed to prevent interaction between multiple transmitters or receivers at the saine end of iiie iiiie. iius each transmitter or receiver is connected i' rough n low impedance path of the coupler to a common line and the coupler isolates the transmitters or receivers one from the other in consequence of the high impedance between the terminals to which these elements :ire coiinffjttd. i

lt is un object of the present invention to provide multichannel bilateral communication over a single transmission line.

lt is another objc-ct of the. present invention to provide t'. bilateral booster nctworlfor siinultt'ineously amplifyanveres L? ing information signals in adjacent frequency bands procceding over a transmission line in opposite directions.

It is yet another object ot the present invention to employ as a booster network in a bilateral transmission system, a four-arm band pass filter bridge having a broad band amplier connected across its conjugate terminals.

It is another object of the present invention to provide a booster network for a multichannel bilateral transmission system employing a broad band amplifier for amplifying signals in all channels and employing a novel arrangement of directional line splitting couplers and band pass filters for preventing ringing of the amplifier.

Yet another object of the present invention is to provide a booster network for a multichannel bilateral transmission system employing a broad band amplifier for amplifying signals in all channels and employing a novel arrangement of properly terminated band pass filters for preventing ringing ofthe amplifier, and for properly terminating the elements of the circuit.

Still another object of the present invention is to provide a booster network disposed in a single coaxial transmission line of a closed loop television system and constituting a four arm impedance bridge having a broad band amplifier connected between its conjugate terminals and employing a novel arrangement of three-terminal, directional, line-splitting couplers connected between adjacent arms of the bridge and further employing band pass filters in the arms of the bridge to provide for proper routing of signals through the network and to prevent ringing of the amplifier.

It is another object of the present invention to provide a multi-channel, bilateral closed loop television system employing a single coaxial transmission line for interconnecting the two ends of thc system.

Yet another object of the present invention to provide a multichannel bilaterai closed loop television system employing a single coaxial transmission line for interconnecting the two ends of the system.

Yet another object of the present invention is to provide a multichannel bilateral closed loop television system which may employ a single coaxial cable with associatcd booster networks for interconnecting the terminal equipments of the system in consequence of a novel arrangement of band pass filters in the terminal equipment and in the booster networks.

Still another object of the present invention is to pro` vide a terminal equipment for closed loop television systems having at least one television transmitter and television receiver tuned to distinct frequencies and employing a novel arrangement of band pass lilters and directional line splitting couplers for preventing overloading of the receiver by the high intensity signals generated by the transmitter.

It is another object of the present invention to provide terminal equipment tor a closed loop television system having at least one television receiver and one television transmitter tuned to distinct frequencies and at least one monitor receiver for monitoring the signals generated by the transmitter and employing directional line splitting couplers and band pass filters for reducing the transmitters signal to a sufliciently low intensity at the terminal equipment to prevent overloading of the video and monitor receivers.

The above and still further obiects, features and advantages of the present Ainvenion will become apparent upon consideration of the following detailed description ot. two specific embodiments thereof, especially when. taken in conjunction with the accompanying dratvings wherein:

Figure i is a schematic circuit diagram of a novci booster network in accordance with the present invention;

Figure 2 is a schematic circuit diagram of a multichannel closed-loop television system employing the network of Figure 1; and

Figure 3 is a schematic wiring diagram of input and output circuits which may be employed with the high and low pass filters of Figures 1 and 2.

Figures 4 to S are schematic wiring diagrams of the low and high pass filter circuits respectively employed in the circuits of Figures l and 2.

Referring specifically to Figure l, of the accompaoying drawings there is illustrated a bilateral booster nctwork 1 for simultaneously amplifying and transmitting in opposite directions video information signals lying in adjacent frequency bands. The bilateral transmission network 1 assumes the form of a four-arm impedance etwork having two input leads Z and 3, the leads 2 and 3 representing long lengths of coaxial cable interconnecting t'ne network 1 with further boosternetworks or television transmitting or receiving equipment not illustrated. The leads 2 and 3 are connected to terminal a of respective directional line splitting couplers i and 5 each having additional terminals b and c. The directional line splitting couplers t and 5 are adapted to provide a low impedance, producing an attenuation of signals of the order of 3 db, between terminals a and b and a and c and vice versa, and to provide a high impedance, producing an attenuation of signals or thc order of 2O db, between terminals b and c and vice versa. The couplers 5 and 4 may be of the type disclosed and claimed in the aforementioned patent application No. 460,256, now Patent No. 2,782,379.

The terminal b of the coupler 4' is connected through a low pass filter 6, illustrated in detail in Figure 4 of the accompanying drawing to a terminal b of a further directional coupler 'l'. Terminal c of the coupler 7 is connected through high pass filter 8 illustrated in detail in Figure 5 of the accompanying drawing, to the terminal c of the coupler 5. crminal c ci the cou pier is connected through a second high pass tiltery, identical with hlter 2, to a termi-o` c* of a still further directional coupler it), and the terminal b of the coupler 1G is connected through a second low pass filter il, identical to filter 6, to terminal. b ot the coupler A broad band amplifier 12 is connected between the terminals a of couplers 7 and It) and is adapted to amplify equal signals having frequencies falling within the band pass characteristic of the low pass filters 6 and ti and the high pass filters 8 and 9.

The unconnected ends of the leads 2 and 3 are connected to distinct terminal equipments as illustrated in Figure 2 of th accompanying drawings which is de-` scribed in detail subsequently. The equipment associated with lead 2 is adapted to transmit video information lying in the frequency band of the filters 6 and 1i and to receive information lying in thefrequency band of the filters 8 and 9. Conversely the equipment associatcd -with lead 3 is adapted to transmit and receive video information lying within the frequency band of the filters 8 and 9 and 6 and 1l respectively. For purposes of example only it is assumed that each terminal equipment includes two television transmitters and t\vo television receivers and that the two transmitters associated with lead 2 transmit, respectively, the frequencies of television ehanncls 2 and 3 and that the tw'o transmitters associated with lead 3 transmit, respectively', the frcquencies of television channels 5 and 6. The filters 6 and 11 have a cutoff frequency of 66 ine/scc. and uroduce an insertion loss of only l db for signals inthe range of frequencies ot` 66 rnc/seo or lower and produces an attenuation of at least 50 db tor signals in a range of frequencies from 76 nie/scc. and higher. Conscquently the filters 6 and j E provide a low attenuation path for signals in channels l and j which are bc low 66 nie/scc. and provide a high attenuation for signals at the frequencies of channels 5 and 6 which are above 76 nic/sec, The filters S and 9 are high pass filters having a low end cut oil frequency of 76 inc/scc. and provide a low insertion loss of l db, for signals at l 76 me./sec. and higher and a high attenuation for signals below this frequency, attenuating signals at 66 mc./sec or lower by 50 db.

Describing the operation of the bilateral network of Figure 1, information signals applied to lead 2 and having frequencies of channels 2 and/or 3 appear at terminal a of coupler 4 and proceed through connector 4 to its terminals b and c.v The signals appearing at terminal b proceed through the filter 6 and receive only 1 db of attenuation while signals appearing at terminal c are subjected to 50 db of attenuation. The signal di` vides between these two paths in accordance with the inverse of the attenuation ratio and the major portion proceeds through filter 6 and the coupler 7 to the amplifier l2, where they are amplified and applied via coupler 1 0, filter 11 and coupler 5 to the lead 3. The total attenuation of the signal by couplers 4, S, 7 and 1t) and filters 6 and 11 is 14 db and if the signal energies are increased 60 db by the amplifier l2, a net gain of 46 db is provided.

The impedance of the filter 6 presented to the line 2 is approximately equal to the characteristic impedance of the line for signals within the pass band of the filter 6. The impedance presented to the line 2 by the filter 9 for signals in its rejection band is considerably greater than the characteristic of the line and might produce serious reflection problems if it were not for the coupler 4. Since coupler 4, as do the other couplers 5, 7 and 1l), divides the signal power in accordance with the inverse ratio of the impcdances presented to the line 2 by the filters 6 and 9, only a small proportion of the input power is divided to the filter 9 and therefore reflection of signals in consequence of the impedance mis match between the line 2 and filter 9 has little effect upon the VSWR of the circuit.

ln the Vein. the impedance mismatch from the filter 6 appearing acr ss the output circuit of filter 9 has little effect on the resultiue impedance presented by line 2 to filter 9 in consequence of the 20 dh attenuation subsisting between terminals b and c of coupler It which appears in series with the. f) db attenuation provided hy the filter 6.

The above analysis of impedance effects of the filters outside or their pass bands applies with equal validity to the other three terminal junctions in the network 1. ln order for the amplifier l2 to present the proper impedance to thc junctions at couplers 7 and 10 the inipedance of the input and output circuits of the amplifier 12 must be matched to the characteristic impedance. of the filters. This matching may be effected by conventional circuits such as matching transformers or attenuation networks.

The signals falling in the frequency range of channels 5 and 6 are applied to the lead 3 and proceed through the couplet' 5 to its terminals ,c and b. The signals appcaring at terminal c proceed through the filter S and are attenuated only 1 db while the signals appearing at terminal b are attenuated 56 db by the filter 11. Consequently, the signals appearing on lead 3 proceed through filter 8, coupler 7, amplifier 12, coupler 1f?, filter 9 and coupler 4 to the lead 2. Since the amplifier 12 is a broad band amplifier the signals applied to lead 3 are amplified to thc same extent as the signals appearing on lead 2 and the net gain for the former signals is 64 db.

The utilization of a single aiiipiifier i2 and consequently a single booster network 1 is possible as a result of effective isolation of the input and output circuits of the amplifier l?. which prevents a ringing of thc network at its operational frequencies. The amplifier l2. in co ion with the filters 9 :ind 6 und filters "it and S constitutesrespectively two closed loop regenerative circuits which will ring; that is, oscillate at tliesignal frequencies, unless the gain of each of these two closed loops is maintained at less than unity. Sufficient attenuation of tht.` signals proceeding through closed loops is achieved in accordance with the present invention in consequence of the high signal isolation attenuations provided by the couplers 4 and 5, and filters 6 and 8. Thus, the signals falling within the pass band of filter 9 and appearing at terminal c of coupler Il, ai'c attenuated 20 db in passing from terminals c and b of coupier 4 and are attenuated another 5() db by the filter 6. This constitutes a total attenuation of 70 db and since the total amplihcation of the signals by the amplifier 12 is only db, the gain ofthe loop is less than unity and the circuit will not ring.

It is apparent from the above description that the booster network l permits multichannel bilateral transmission of information signals in adjacent frequency bands. The ultilization of the directional connectors it, 5, 7 and l@ and the filters 6, S, 9 and il effects proper routing of the signals of different frequencies through low impedance paths in series with the unitary amplifier 12 and prevents ringing of the network. Consequently, the network provides a transmission system utilizing a minimum of circuit components and a single amplifier to achieve multichannel bilateral transmission over a single coaxial transmission line. The high signal gain achieved by the network 1 permits it to be placed intermediate long lengths of transmission lines and minimizes the totality of equipment required in any given transmission path length thereby materially reducing the cost of the equipment over prior art apparatus.

The network 1 is particularly adapted for employment in a closed loop multichannel television circuit employing multiple television transmitters and television receivers at each terminal equipment. Referring specifically to Figure 2 of the accompanying drawings which illustrates a bilateral multichannel closed loop television system employing the network l of Figure l, a rst terminal equipment i3 comprises television tiansmtters I4 and .f5 which for example only, transmit information at the frequencies of video channels 2 and 3 respectively and television receivers f6 and 17 which for example receive information at the frequencies of video channels 5 and 6. The first terminal equipment 13 is connected via coaxial lead Z, the bilateral network 1 and the coaxial lead 3 to a second terminal equipment f8. The

. terminal equipment 13 includes television receivers 19 and 20 adapted to receive information at the frequencies of television channels 2 and 3, respectively and transmitters 21. and 7.2 for transmitting information at the frequencies of television channels 5 and 6 respectively. information signals generated by transmitters 14 and 15 proceed from left to right as viewed in Figure 2 and are received by receivers i9 and 2G, respectively, while information signals generated by transmitters 2 and 22 proceed from right to left as viewed in Figure 2 and are received by receivers 16 and 17, respectively. Consequently, the apparatus illustrated in Figure 2 permits two, distinct bilateral transmissions (both video and audio) to be carried on simultaneously.

In order to effectively isolate the various elementsl of each terminal equipment from one another. n network employing directional couplers and low and high pass filters is utilized. More specifically the transmitters 14 and 1S are connected over coaxial lends 23 and 2st to terminals b and c of u directional coupler 2S. 'Terminal ri of the coupler 2S is connected viav leid 26 low pas filter 27 and lead 2S to terminal c of a directional coupler 29 and through the directional coupler .'59 to the lead 2. The filter 27 is identical to filters 6 and il of Figure l. The coupler 25 provides :i ffl dh atteint` ation between terminals l1 and c :ind therefor: cti: t l isolates the transmitters alt and l5 from one' another. The receivers iti and l'/ are connected respectively over leads 30 and 3l to terminals b and e of n directional coupler 32, and terminal n of coupler 3?, is connected vin lead 33, high pass filter 3i, und lead 35 to teriiiinui b of the directional coupler 29 and through coupler 29 t the lead 2, the filter 34 being identical to filters 8 and 9 of Figure l. The coupled 32 provides 2O db of attenuation between receivers 16 and 17 and thereby effectively isolates one from the other while the coupler A29 and high pass filter .3a-prevent the receivers 16 and 17 from being overloaded by the signals generated by the transmitters 14 and 15. v

The signals generated by the transmitters 14 and 15 are applied to a long length of coaxial cable; for example 2000 feet, represented by lead 2 and must be of sufficient intensity to be distinguishable over background noise at the network 1. On the other hand the signals applied to the receivers 16 and 17 have just traveled over the same length of eable2 from the network 1 and are of quite low intensity. Consequently, the gain in the tuner stages of the receivers 16 and 17 must be quite high and the receivers would become overloaded by the signals generated by transmitters 14 and 15 in the absence of specific circuitry for attenuating the latter signals. This circuitry includes the coupler 29 which produces a 2() db attenuation of the signals generated by transmitters 14 and 15 and which appear on lead 35 and the filter 34 produces a further attenuation of these signals of 50 db. Consequently, a total attenuation of 70 db is provided and the channel 2 and 3 signals appearing on lead 33 are of a sutiiciently low intensity to be tuned out by the receivers 16 and 17. However, the channel 2 and 3 information signals appearing on lead 33 are of'a sufficient energy level to be detected by appropriately tuned receivers which receivers may be employed as monitors for the transmitters 14 and 15. In order to provide such a monitoring system the lead 33 is connected to terminal a of a further directional coupler 35 and the terminals b and c thereof are connected respectively to a channel 2 receiver 37 and a Il r this arrangement the. coupler 29 and high pass tilter 34 which prevent the receivers 16 and 17 from being overloaded by the signals generated by transmitters 14 and 15 also serve to attenuate these signals sufficiently to permit the utilization of local monitors or receivers 37 and 38 which pr vide for both visual and oral monitoring of the information transmitted by transmitters 14 and 15 respectively.

The terminal equipment 18 may employ monitors 39 and 40 and is identical with equipment 13 except that the high pass and low pass filters are interchanged. Thus a high pass filter 41 is included in the transmitter circuit between transmitters 2.1 and 22 and the coupler 43. The filters 27 and 41 of equipment 13 and 1S serve the identical function of routing the incoming signals to the receiver channels by providing a high attenuation in the transmitter circuits at the frequencies of the incoming signals.

As in the circuitry of the booster network 1, the directional couplers 29 and 43 serve not only as elements for attenuating signals, but in their capacity of power dividers they effect proper termination of the lines 2 and. 3 and the transmission circuit filters 27 and 41.

It is apparent from the preceding description that the apparatus of the present invention provides a highly liexiblc, closed loop television system capable of multichannel bilateral communication over a single transmissionline. The utilization of n single transmission link between the terminal equipments effects a considerable saving of expensive coaxial cable and broad band, amplifiers. Further the terminal equipment and the net@ work 1 employ a small number of substantially identical elements which minimizes fabrication costs.

The system of the present invention has been described as employing only tuo channels of transmission in each direction but it is within the scope Of the present invention to employ additional channels tor each direction of transmission. Thus, transmission may occur in one direction on channels 2 to 6 and transmission in the other direction over channels 7 to i3, this division of channels 1G simplifying design of. the low and high pass filters since the frequency bands transmitted in the two directions are widelyv separated. The additional transmitters and receivers can be connected to the receiver and transmitter circuits in the same manner the monitors 37 and 38 are connected to lead 33` The system of the present invention as thus far described has been limited to circuits employing directional line-splitting couplers in both the booster network 1 and in the terminal equipments 13 and 18, ln a second embodiment of the invention the directional couplers may be omitted and the filters redesigned to effect the functions previously provided by the couplers. The couplers are employed in the circuitry of Figures l and 2 of the accompanying drawings in order to attenuate the signals around the loops provided by filters 6 and 9 and amplifier 12 and filters 8 and 11 and amplifier- 12 sufficiently to prevent ringing of the circuits and were further employed to take advantage of their power dividing'properties so as to properly terminate all elements of the circuit.

The former function of the couplers may readily be assumed by the filters by increasing their attenuation of signals outside of their pass band. {efcrring to the exemplary figures employed in the discussion of the circuitry of Figures l and 2, the filters would be required to provide an additional 20 db of attenuation upon removal of the couplers; that is, each filter would attenuate signals outside of its pass band by db.

Modifications of the filters to efleet proper termination of the lines requires redesign of their input and output circui.

A type of low and high pass filters which may be employed in the circuits pf Figures l and 2 are illustrated in Figures 4 and 5 respectively while the identical filter input and output circuits required in an appara is not flying the couplers Culp; accompanying drawings.

Referring specifically to Figure 4 of the accompanying drawing the low pass filter comprises series m sections including for purposes of example only three sections defined by three series connected inductors -l-i, i5 and and four complex shunt impedances 'l-7, $8, 49 and Sil each having a capacitor and induetor connected in series. The design of this low pass filter is conventional'and the design equations which must be employed to obtain the values of the various elements in order to efiect the desired band width and attenuation of signals outside of its pass band are wel known in the art. The series connected induetors 4-1--46 are shown connected between two terminals b which as will appear subsequently refer to the terminals b" of Figure 3 of the accompanying drawings.

Referring to Figure 5 of the accompanying drawings, there is illustrated a high pass filter of the series m type. The high pass filter includes for the purposes of illustration only, three series m sections and comprises three series connected capacitors 51, 52 and 53 and four complex shunting impedunces 54, 5S, 56, and 57 cach having a series connected capacitor and inductor. The capacitors 51-53 are illustrated as connected in series between two terminals c either of which corresponds to the terminal c" of Figure 3.

As in the case of the low pass filter of Figure 4, the high pass filter of Figure 5 is of conventional design and the equations for determining the values of the various elements to obtain desired characteristics are well known.

Referring now to Figure 3 of the accompanying drawings, there is ill istruted a network which comprises a series m half section for properly terminating the contini lines und the iiircrs in equipments wherein the directional couplers are not cmpioyeti. The network includes a capacitor 58 connected between terminals a and c, the terminal a being connected to a coaxial iine and thc terminal c being connected to one of the terminals c of Figure 5. Connected between the terminals a and b is an induciai Firmin'. '2 -..l fr. 1.D..n. lo

tor S9, the terminal b being connected to one of the terminals b of Figure 4. A series circuit comprising a capacitor 69 and an inductor 6l is connected between the terminal a and a point of reference potential, such as ground, to which the outer conductor of the coaxial cable is also connected. The series circuit therefore is con nccted across the coaxial cable.

As previously pointed out, the elements 58, 59, 6i) and 6l constitute a series m half section which may be considered as being derived by eliminating the first or last, as the case may be, shunt impedance of the respective series in high and low pass filters and further by connecting a shunting impedance across the coaxial line.

When the filter circuits of Figures 3, 4 and 5 are arranged in a booster network 1 the circuit of Figure 3 constitutes the output circuit of one and the input circuit of the other of the filters of Figures 2 and 3. Thus if the terminal a of Figure 3 is connected to the coaxial line 2 of Figure l or 2 the circuit of Figure 3 constitutes the input circuit of the low pass filter 6 and the output circuit of filter 9. With the terminal n appropriately connected to the input circuit of the amplifier 12 the circuit of. Figure 3 constitutes the output circuit of both filters 6 and 3 and if terminal a is appropriately connected to the output circuit of the amplifier 12, the circuit of Figure 3 constitutes the input circuit of filters 9 and 1l. ln all of these arrangements, each of the filters and the coaxial lines are coupled to circuits having the proper input or output impedances in their pass band, this impedance being the midshunt impedance of a series m derived section and by proper design this impedance may be made to equal the characteristic impedance of thc coaxial lines.

Although the discussion of the circuits of Figures 3-5 has thus far been restricted to the booster network. 1, the circuits, particularly of Figure 3 may be employed in thc terminal netti-'Orks of 13 and 1S of Figure 2. Either or both of the couplers 29 and s or this latter figure iii-ay be replaced by the circuit of Figure 3 to provide proper termination of the filters 27 and 34 and line 2 or filters 41 and 42. and line 3.

It is apparent from the above discussion that the system of the present invention provides a bilateral, multichannel communication network of highly flexible design which may employ various types of circuits and may operate in various portions of thefrequency spectrum. Although the invention is described as particularly applicable to closed loop television systems, it is not intended to limit the scope of the invention to one particular field of communications since the principles of circuits set forth herein may be employed in various types of communication systems wherein bilateral, multichannel communication between remote locations over a single circuit is desired.

While I have described and illustrated one specified embodiment of my invention, it will be clear that variations of the general arrangement and of the details of coristruction which are specifically illustrated and described may be resorted to without departing from the true spirit and scope of the invention as dened in the appended claims.

What is claimed is:

l. A bilateral coaxial cable transmission line including a booster network for simultaneously transmitting in opposite directions over a conductive line, information contained in modulated independent carriers occupying separatf` broad Frequency bands, said booster network comprising a four-arri impedance bridge having two pairs of opposite iii-nis. two signal terminals and two conjugate terminals, first band pass means adapted to pass infor tation in one olf said frequency bands only, means con une of salti iii'st band pass mtnrs in each of s arms ot ont` or said pair ot opposite arms of said bridge network` second bund pass means adapted to pass information in the other of said frequency bands only, means connecting one of said Second band pass means in cach of the said arms of the other pair of opposite arms, directional coupler means at cach terminal of the network providing a high effective attenuation between adjacent arms of said bridge network and a low effective attenuation between said arms and said signal and conjugate terminals, a single broad band amplifying means for amplifying information in the two frequency bands, means connecting said amplifying meansv between said conjugate terminals, and means conmeeting each of said signal terminals to a coaxial cable through the directional coupler means at the signal terminals, said directional coupler means providing sufficient attenuation, together with the attenuation of said band pass means, to keep below unity thc gain in the closed loop formed by said bridge while permitting high gain of the respective frequency bands in opposite directions through said bridge.

2. The combination in accordance with claim l, wherein said coupling means comprise a three terminal network providing a low effective attenuation between a first terminal and the other two terminals and a high effective attenuation between said other two terminals and means connecting said other terminals to said bridge arms.

3. The combination in accordance with claim l, wherein each of said band pass means is effective to pass information in at least two adjacent television ciannels distinct from the two adjacent television channels passed by the other of said band pass means.

4. The combination in accordance with claim 1, ther comprising at least one television transmitter and one television receiver associated with each of said input leads, one of said transmitters and one of said receivers associated with different input leads being adapted to operate in one of said frequency bands and the other of said transmitters and receivers being adapted to operate in the other said frequen bands, further directional coupling means providing a rcmttvcly low attenuation between each of said input leads and its associated transmitter and receiver and providing a relatively high attenuation between its associated transmitter and receiver, and further band pass means coniiected between each of said impedance means and iis associated transmitter and receiver and providing a relatively high attenuation therebetween, said band pass means providing a relatively low attenuation path. for information lying only in the frequency band of its associated transmitter or receiver.

5. The combination in accordance with claim 4 having at least two transmitters and two receivers associated with each of said input leads, the transmitters associated with one input lead operating in distinct por tions of one of said frequency bands and the receivers associated with the one input lead operating in distinct portions of the other of said frequency bands, and additional directional coupling means providinnl a relatively low attenuation between said receivers and their associated band pass means and between said transmitters and their associated band pass means.

6. The combination in accordance with claim 5, furthei' comprising at least one additional television receiver, and means connecting said additional television receiver between said further television receiver adapted furl to operate in the sume frequency band as the transmitinput lead as the latter .means connecting said two first filter ieans at least four, three terminal directional line splitting couplers each providing a low attenuation between iirst and second and first and third terminals and a high attenuation between said second and third terminals, two parallel transmission paths cach comprising a-scrics circuit including one of said first filter means, one of. said second iilter means and one of said directional couplers having its second and third terminals connected between said filter means, a broad band amplifier for amplifying signals in both of said frequency bands, means connecting said amplifier'. between first terminals of said directional couplers connected in said series circuits, at least two input leads, means connecting each of said input leads to a first terminal of a distinct directional coupier, means connecting one of said first and second filter means to the second and third terminals ot one oi said latter directional couplers and means connecting the other of said first and second fi ter means to the second and third terminals of the other of the latter of said direction couplers.

8. A coaxial cable transmission line comprising bilateral transmission netwoizi; for simultaneously transmitting in opposite directions over a conductive 'zine information in adjacent frequency bands, said transmission network comprising, at least two first litter means for passing information in one of said frequency bands only, at least two second filter means for passing information in the other of said frequency bands only, a plurality of three-terminal directional couplers, each providing a low attenuation path between a first ter minal and the other two terminals and a high attenuation path between the other two terminals, a four arm bridge network having two signal terminals and two' conjugate. terminals, a broad band amplifier connected between said conjugate terminals of said bridge network, in a one pair ii-opposite arms of said bridge network and cunneetiug said. two second filter means in the other pair of opposite arms of said bridge network, one of said directional couplers being connected at each junction of said `arms, and means interconnecting said arms and said directional couplers to provide high attenuation paths between said arms and low attenuation paths between said ainplitiers and its associated arms and coaxial cable line sections respectively connected to said signal ter- Initials;

9. A bilateral booster network Ifor simultaneously transmitting in opposite directions over a conductive line, information iii adjacent frequency bands, said booster network comprising a four-arm impedance bridge having. two pairs of opposite arms, two signal terminals and two conjugate terhiinals, first band pass means adapted to and said lilters'ifapproximately in their characteristic im` pedances, broati band amplifying meansv for ainpi'fying information in the two frequency bands and means for connecting said amplifying means between said conjugate terminals over coaxial leads.

lO. I`he combination in accordance with claim 9 wherein said filters comprise liz-derived filters and wherein said means for terminating each comprises-a four terminal half section of anim-derived filter section having two terminals connected across its associated lead and one of each of the remaining terminals connected to a different one of its associated filters. .i

ll. The invention according to claim 9, the impedanees of said filters being so arranged that each of said filters in its rejection hand provides the in'ipedance 'required by the other filter in its pass band to present tite eharae teiistic impedance to said coaxial' line.

References Cited in the file of this patent UNITED STATES PATENTS qw., i 

